Spark plug



Patented Jan. 6, 1942 SPARK PLUG Charles B. Sawyer, Cleveland Heights, Ohio, as-

signor to The Brush Beryllium Company, Cleveland, Ohio, a corporation of Ohio No Drawing. Application April 30, 1940, Serial No. 332,479

3 Claims.

This invention relates to spark plug insulators composed of pure beryllium oxide. It relates especially to spark plug insulators particularly adapted for use in aircraft engines.

It is an object of this invention to provide a spark plug insulator having especially high thermal conductivity.

It is another object to provide a spark plug insulator having high electrical resistivity and good strength at high temperatures.

It is another object to provide a spark plug insulator composed of beryllium oxide characterized by a softening point which lies above about 2000 C.

It is another object to provide a gas-tight electrical insulator composed of pure beryllium oxide and characterized by a low absolute porosity, high thermal conductivity, high electrical resistivity and good hot-strength.

There has long been a need for a refractory spark plug insulator having high thermal conductivity. The need has been felt especially in the aircraft industry where spark plug insulators having this property have become more and more necessary as more powerful engines have been contemplated and/or built. The need was pointed out in 1919 in Report No. 51 of the Annual Report of the National Advisory Com mittee on Aeronautics, and again in 1934 by H. Navratiel in vol. 37 of Automobiltech. Z. pgs. 238-40. The insulators available heretofore for such use have had relatively low thermal conductivity and as a consequence have had such short and uncertain life as to militate against the development of more powerful motors. Many expedients have been resorted to for the purpose of overcoming the difficulties resulting from the use of spark plug insulators having low thermal conductivity.

The difiiculties referred to will be more apparent through a consideration of the service conditions to which an aircraft engine spark plug insulator is subjected. Fundamental, of course, is the fact that practically all aircraft engines have been of the air-cooled type, this being generally the result of a desire to attain a high horsepower to weight ratio by avoiding the use of the necessarily large volume of heavy cooling liquid which would be required if the engines were cooled by means of such medium. Eflicient air-cooling requires extensive radiating surface and the use of materials having a high thermal conductivity. As the power rating of air-cooled motors is increased, the need for more and more efiicient air-cooling becomes necessary pansion, gas-tightness, strength, etc., are con cerned, they all have had poor thermal properties and with a few exceptions, have had relatively poor electrical resistivity and strength at high temperatures. Spark plug porcelains in aircraft engines are subjected to severe conditions which involve high temperatures and stresses and sudden heating and cooling effects. Thus the insulator may be subjected to gas pressures over 600 lbs. per sq. inch; in addition, the gases which contact the insulator alternate rapidly in temperature from 0 C. to around 2500 C. and they tend to deposit soot. Through all this the porcelain should remain a good electrical insulator even at its maximum temperature of around 1000 C. This requires strength, toughness, high resistivity at high temperatures, a reasonably constant thermal expansion, material which does not undergo permanent volume changes and a material which does not catalyze the reactions causing soot deposition.

In accordance with this invention, I provide a spark plug insulator composed of pure beryllium oxide which has a high thermal conductivity and which also meets the other requirements to a remarkable extent. I recognize that many ceramic compositions containing beryllium oxide have already been proposed for use in spark plug insulators. Some of these compositions have included only minor amounts of beryllium oxide, while in a few, the beryllium oxide content has been a major part of the whole composition. An example of the latter type, is the composition disclosed by Reichrnann in U. S. Patent No. 2,033,300. Beryllium oxide is the main constituent of this composition, but additions of other refractory oxides are made thereto for the purpose of reducing the sintering point or softening temperature of the resulting mixture. These additions impair the hot-strength of the composition and furthermore impair the thermal conductivity and electrical resistivity. Insulators provided according to my invention avoid such additions and their attendant disadvantages and accordingly have good hot-strength, high thermal conductivity, high electrical resistivity at high temperatures and have the mechanical properties inherent in beryllium oxide undiminished by the presence of foreign oxide additions. Such insulators have, moreover, long life, great resistance to thermal shock, desirably weak catalytic properties, and a tendency to toughen during use.

Pure beryllium oxide as contemplated by this invention is beryllium oxide which does not sinter at temperatures below about 1700 C. and which is preferably more than 99.9% pure after ignition. A very good practical testfor adequate purity of the beryllium oxide used is its apparent bulk density after being heated for 'the first time for a period of ten minutes at a temperature of 1600 C. in an atmosphere not more strongly reducing than that of hydrogen. This test is fully described in the U. S. Patent No. 2,176,906, gran'ted October 24, 1939 to C. B. Sawyer and B. R. F. Kjellgren. Suitable material for use in spark plug insulators has a maximum apparent bulk density of about .35 as determined in accordance with this test. When such material is used, it is possible to produce a spark plug insulator having a thermal conductivity of at least 300 to 400 per cent greater than that of ordinary insulator materials now in use, and in some instances, the conductivity may exceed 500 to 600 percent more than that of such materials. These high values .of conductivity have been determined through tests which have compared silicon carbide, fused alumina, fused magnesia, beryllium oxide, and typical insulator porcelains such as are now employed in spark plugs and many other materials. These tests were effective in determining the relative values of thermal conductivity exhibited by these various materials. On the basis of such tests, I have determined that beryllium oxide exhibits a thermal conductivity approximating that of silicon carbide, and has a thermal conductivity in excess of the conductivity of fused zirconium oxide, fused aluminum oxide or fused magnesium oxide.

It will be understood that by reason of the fact that beryllium oxide insulators forming the subject matter of this invention exhibit such improved values of thermal conductivity, such insulators are capable ofv dissipating heat at an appreciably greater rate than in the case of other refractories now used as spark plug insulators. This greater rate of heat dissipation results in smaller temperature differentials within the body of the insulator, thereby reducing the internal strains which result from differential thermal expansion. Furthermore, by reason of the high thermal conductivity, the insulator attains a relatively lower operating temperature so that the total expansion is reduced. This fact is of value since it permits spark plugs to be manufactured with a crimped joint between the metal shell of the spark plug and the ceramic insulator. B reason of the fact that the pure beryllium oxide insulator operates at a generally lower temperature than in the case of other materials, and by reason of the great resistance to thermal shock exhibited by pure beryllium oxide, there is less tendency for the insulator to crack or spall as a result of the rapid alternalions in temperature which occur, as pointed out above, in the engine gases. Furthermore. by reason of the lower operating temperature, the dielectric strength is maintained at a higher value and in addition, the lower operating temperature reduces the tendency of the engine gases to deposit carbon or soot. It should be recognized in this connection that the deposition of soot is a function not only of the temperature of the insulator but also of the catalytic effect of the insulator. As previousl pointed out, pure beryllium oxide has a weak catalytic effect upon the reactions which cause soot deposition, so that soot deposition from this source is slight to negligible. In addition, the lower operating temperature also minimizes the deposition which occurs as the direct result of the temperature of the insulator. All of the foregoing effects and advantages resulting from the use of pure beryllium oxide contribute to the effect of reducing the tendency for preignition to occur. Furthermore, they promote a longer life not only of the insulator but of the spark gap electrodes and of the metal parts of the spark plug body. The ultimate result of the higher thermal conductivity of beryllium oxide is an insulator which is far more durable than those composed of other materials.

Spark plug insulators composed entirely of pure beryllium oxide can be made in accordance with the usual processes for handling the common insulator materials with the exception, however, that higher sintering temperatures are required and with the further exception that the beryllium oxide should be prefired at above 1700 C. for the purpose of shrinking it and raising its absolute density. Preferably the beryllium oxide should be prefired at a temperature around 2000 C. or higher, since it has been found that such temperatures result in material which undergoes reduced shrinkage during the subsequent firing steps. Such prefiring temperature may include fusion at around 2570 C. Thus material having an apparent bulk density of about .35 after firing at 1600 C. in accordance with the test outlined above, will have an apparent bulk density of around .95 after it has been prefired at temperatures between 2000 C. and the melting point. It will be seen that the apparent density has been increased more than two and one-half times by firing at the higher temperature of 2000 C. .The prefired material may be molded in the usual manners after either wet or dry milling and with or without the application of pressure during the molding step. Thus, with material which has been wet milled in accordance with recognized practice which is effective in providing the proper proportions of coarse and fine particles, a slip may be produced which may be cast directly into the desired shape of insulator. Such slip-cast material may be sintered at the necessary high temperatures (around 2000 C.) without cracking or undergoing excessive shrinkage. Presintered berryllium oxide which has been dry milled and appropriately sized with respect to coarse and fine particles may be compacted in a suitable mold with or without the aid of vibratory compacting devices and then subsequently compressed under high pressure. Such compacted and compressed material may be subsequently sintered without cracking and without undergoing as great shrinkage as in the case of wet milled material. In using either process, the final sintering should preferably be conducted at temperatures above 1900 C. and should be continued for relatively long periods of time, since it has been found that the long sintering period is effective in promoting low absolute porosity in the insulator and in thereby attaining the desirably high thermal conductivity which is inherent in pure berryllium oxide. The prolonged sintering period also .in an electric arc.

toughens the insulator and improves its resistance to thermal shock.

Spark plug insulators made in accordance with the process outlined above should preferably be glazed to insure complete gas-tightness and also to provide as smooth a surface as possible. When it is desired to effect such glazing without using the customary easily-fusible glazing compositions, it may be accomplished conveniently in the flame of an oxyhydrogen torch, or An exposure to either of these heating media causes suiilcient superficial fusion at the surface of the insulator to cause the desired glazing action. Those skilled in the art will understand that beryllium oxide may be reduced to metallic beryllium at the temperatures employed in this glazing step if a reducing atmosphere prevails. To avoid such reduction, neutral or oxidizing conditions should be maintained adjacent the spark plug insulator which is being glazed.

Having now disclosed my invention, what I claim is:'

l. A spark plug insulator composed of highpurity beryllium oxide having an apparent bulk density not in excess of about .35, said insulator being characterized by high thermal conductivity and a softening point above about 2000 C.

2. In a spark plug, a ceramic insulator composed of high-purity beryllium oxide having an apparent density not in excess of about .35, said insulator being characterized by a thermal conductivity at least 300% greater than that of insulator porcelain, a softening point above about 2000 0., high electrical resistivity and good hotstrength.

3. In a spark plug, a ceramic insulator composed of beryllium oxide having a purity of at least 99.9%.

CHARLES B. SAWYER 

